1. Field of the Invention
This invention relates to an electrical substrate material particularly useful as a circuit or wiring board substrate material and the printed wiring boards formed therefrom. More particularly, this invention relates to a new and improved printed wiring board substrate comprised of a ceramic filled, microfiberglass reinforced fluoropolymer composite material which exhibits improved electrical performance over other printed wiring board substrate materials; and exhibits low coefficients of thermal expansion and relatively high compliance resulting in improved surface mount and plated through-hole reliability. The high degree of compliance of such a highly filled composite is an unexpected result. The ceramic filler is coated with a material (preferably silane) which renders the surface of the ceramic hydrophobic as well as providing other important and unexpected features. This ceramic filled, microfiberglass reinforced fluoropolymer composite material is also well suited for use in an improved packaging system for integrated circuit chips.
2. Description of the Prior Art
The performance of high speed computers is becoming increasingly limited by current methods of interconnection, from the printed wiring board (PWB) level on up. As ECL or GaAs logic devices are developed with high operating frequencies and shorter rise-times, the PWB substrate, particularly the material comprising the substrates, becomes an important component of circuit design, and substrate material selection is critical. Presently, two major issues for substrate selection are: (1) electrical performance; and (2) surface mount reliability.
For improved electrical performance, and lower dielectric constant, lower loss PWB materials are required, with these properties being stable over wide frequency and temperature ranges. PWB substrate materials currently used which provide reliable electrical performance for high speed applications include fluoropolymer (PTFE) glass composite materials.
In order to use surface mount technology reliability, a close match of the coefficients of thermal expansion (CTE) of the electronic package and the PWB substrate is required to reduce stress on solder joints. However, since the package and the board usually heat up at different rates, a close CTE-match may not be enough. A compliant (low modulus) substrate minimizes the stress on the solder joint resulting from differential strain between the package and the board. Such substrate materials demonstrating favorable coefficients of thermal expansion in the X-Y plane include polyimide/quartz and polyimide/KEVLAR composite materials.
However, no prior art approach has successfully provided a PWB substrate which has effective electrical performance as well as surface mount reliability. While PTFE/glass composites have excellent electrical properties, these materials have poor dimensional stability leading to poor surface mount reliability. Conversely, while polyimide/quartz and polyimide/KEVLAR have excellent surface mount reliability characteristics, these materials have relatively high dielectric constant and high loss (or dissipation factor) leading to poor electrical performance. In addition, no prior art approach has a Z-direction thermal expansion coefficient that is sufficiently close to that of copper so that high reliability plated through hole interconnections are achieved.
U.S. Pat.. No. 4,335,180, assigned to the assignee hereof, all of the contents of which are incorporated herein by reference, discloses an electrical substrate material which has both improved electrical properties and surface mount reliability. This material is described as a microwave circuit board comprised of a glass reinforced fluoropolymer filled with a ceramic. While suitable for its intended purposes, the microwave material of U.S. Pat. No. 4,335,180 suffers from an important deficiency in that the material exhibits a high degree of water absorption (i.e., the material is hydrophilic). As a result, moisture is absorbed into the microwave circuit leading to undesirable changes in the electrical and other properties of the circuit. Also, this material suffers from other drawbacks including low strength, poor copper adhesion and poor dimensional stability.